The present invention relates generally to a zoom lens and an electronic imaging system using the same, and more particularly to a less costly electronic imaging system inclusive of video or digital cameras, the depth dimension of which is diminished by providing some contrivance to an optical system portion such as a zoom lens, and which ensures zoom ratios high enough to cover a wide focal length range from a wide-angle end to a telephoto end.
In recent years, digital cameras (electronic cameras) have received attention as the coming generation of cameras, an alternative to silver-halide 35 mm-film (135 format) cameras. Currently available digital cameras are broken down into some categories in a wide range from the high-end type for commercial use to the portable low-end type. In view of the category of the portable low-end type in particular, the primary object of the present invention is to provide the technology for implementing easy-to-use, high-zoom-ratio, less costly video or digital cameras whose depth dimension is reduced while high image quality is ensured, and which have zoom ratios high enough to cover a wide focal length range from a wide-angle end to a telephoto end.
The gravest bottleneck in diminishing the depth dimension of cameras is the thickness of an optical system, especially a zoom lens system from the surface located nearest to its object side to an image pickup plane. To make use of a collapsible lens mount that allows the optical system to be taken out of a camera body for taking and received therein for carrying now becomes mainstream.
However, the use of the collapsible lens mount is not preferable in view of ease of operation, because much time is taken for sending the zoom lens received at it ready for use. Making the lens group nearest to the object side movable is again not preferable for prevention of entrance of moisture and dust.
More recent years have seen a camera version that takes no waiting time for sending it ready for use (for booting a zoom lens up), works favorably for prevention of entrance of moisture and dust and is much more slimmed down in its depth direction by use of an optical system with an optical path bent by a reflecting optical member such as a mirror or a prism. In a typical camera of this version, the lens group nearest to the object side of the zoom lens is fixed in terms of position and the reflecting optical member is received in that lens group, so that the subsequent optical path is bent longitudinally or transversely with respect to a camera body, thereby reducing the depth-direction dimension as much as possible.
For the time being, most video or digital cameras under the portable category to which the invention is directed would have a zoom ratio of about 3; however, cameras having a zoom ratio of as high as about 5 will come out anyway.
Still, a problem with an increased zoom ratio is that even with an optical path-bent zoom optical system, any compact imaging system cannot be set up, because the amount of movement of lens groups for zooming becomes too large, resulting in size increases in the optical path-bent direction.
For instance, Patent Publication 1 discloses a zoom lens relying on an optical path-bent optical system with a zoom ratio of about 5. However, this zoom lens, albeit having a zoom ratio of as high as about 5, has demerits of using some considerable lenses and being costly, because of a complicated lens contraption comprising six lens groups.
For instance, Patent Publications 2, 3, 4 and 5 disclose an arrangement wherein an optical path is bent by a reflecting optical member located at or between lens groups positioned in the rear of the lens group located nearest to the object side. However, a problem with this is that the depth-direction thickness is hardly reduced after optical path-bending because of an increased distance from the entrance surface to the optical path-bending reflecting optical member.
In addition, a mere combination of the optical path bending relying on a reflecting optical system located in the lens group nearest to the object side as in Patent Publication 1 with that relying upon a reflecting optical member located at or between lens groups in the rear of the lens group located nearest to the object side as in Patent Publications 2, 3, 4 and 5 would not always lead to high-zoom-ratio, compact arrangements.
Patent Publication 1
JP (A) 2004-37967
Patent Publication 2
JP(A)10-20191
Patent Publication 3
JP(A)2000-187159
Patent Publication 4
JP(A)2000-187160
Patent Publication 5
JP(A)2002-169088